Optical fibers are essential components in modern telecommunications systems. Comprised of thin strands of glass, optical fibers enable the transmission of light, or optical signals, over long distances with little loss. An optical fiber typically has a core of glass having a specific index of refraction surrounded by a glass cladding having a lower index of refraction. Thus, light entering the fiber is retained within the core by internal reflection.
In applications wherein a single optical fiber carries signals on more than one wavelength, such as in wavelength division multiplexing (WDM), fiber Bragg gratings are used for controlling the specific wavelengths of light within the fiber. Fiber Bragg gratings also have other applications such as in fiber lasers. A typical Bragg grating comprises a length of optical fiber having periodic modulations in the index of refraction in its core, spaced equally along the length of the grating.
Several methods have been developed to fabricate fiber Bragg gratings. For example, the holographic, or interferometer method uses the interference patterns created at the intersection of two coherent light beams to induce index modulations directly in the optical fiber. A second fabrication method involves the use of a phase mask positioned close and parallel to an optical fiber on which the grating is to be formed. For example, by placing the phase mask between a coherent light source, such as a laser, and an optical fiber, the diffraction caused by the mask replicates the function of an interferometer, creating a plurality of divergent light beams that interfere with each other in a predictable pattern, resulting in periodic alterations in the refractive index of the exposed core of the optical fiber. Typically, fiber Bragg grating fabrication using a phase mask requires stripping away the cladding before exposure of the core, although gratings can be printed onto an unstripped fiber having a cladding that is transparent to the wavelength of light passing through the phase mask.
Because of their ability to selectively reflect specific wavelengths in a narrow bandwidth, while allowing the remaining wavelengths to pass essentially unimpeded, Bragg gratings are used as filters, stabilizers, dispersion compensators and for other applications in fiber optic systems. It is desirable, however, under certain circumstances to broaden the range of wavelengths affected by a Bragg grating. To accomplish this, a technique known as chirping is applied wherein the spacing between the periodic modulations in the refractive index (pitch) of an ordinary Bragg grating is gradually increased or decreased along the length of the grating. Thus a chirped fiber Bragg grating has a wider active bandwidth and a wavelength-dependent time delay because it has a wider range of spacings.
Although the characteristics of chirped fiber Bragg gratings are desirable, fabrication of such gratings has proven difficult and time-consuming. Particularly, the fabrication of chirped phase masks has been challenging. For example, a typical chirped Bragg grating phase mask may have an array of between 100 to 200 grating segments, each between 0.5 and 1.0 mm in length and with a pitch change between segments measured on the picometer scale. The pitch of each successive segment varies continuously from, and must be “stitched” or placed precisely relative to, the preceding segment. Conventional lithography tools such as an electron beam (e-beam) tool (for example, the MEBES III or MEBES 4500, both manufactured by Applied Materials, Inc. of Santa Clara, Calif.) have been unable to achieve the accuracy necessary to produce a phase mask for a chirped Bragg grating in a time period that makes them competitive.
A lithography tool uses an image writing element such as a laser beam or an electron beam to print an image, such as that of a phase mask onto a substrate. Thus exposed, the substrate can be processed such that a grating pattern comprising, for example, an array of alternating lines and spaces is etched into the substrate. On a MEBES tool in particular, phase mask fabrication has been attempted using a “scale factor” approach and is therefore particularly complex. Specifically, a basic unscaled segment, or grating pattern, is established having a predetermined address unit defining its size. The grating pattern is rescaled as needed by applying scale factors to the address unit. Scale factors are dimensionless values that are applied by the MEBES tool to the address unit to achieve the desired reduction or magnification of the grating pattern. When applied across an entire mask, a specific chirp, or rate of change in the grating period of the finished phase mask is the result.
FIG. 1 is a block diagram of the steps in fabricating a chirped fiber Bragg grating from a phase mask produced applying the scale factor method on lithography tool such as a MEBES tool which uses an electron beam as an image writing element. The first step, shown as 10, is to provide a photoresist coated substrate. As is common in the art, the substrate is often approximately 152×152×6.35 mm and is typically formed of amorphous quartz, such as fused silica, or other similar material, due to its ability to transmit ultraviolet light, or of some other substantially transparent material. One of the major surfaces of the substrate is typically coated with 3000 to 5000 angstroms of photoresist material such as PBS or ZEP7000 over 1000 angstroms of an opaque layer, comprising, for example, chrome or other similar materials. The opaque layer may also be coated with an anti-reflective layer, such as a chrome-oxide layer or a layer of other similar materials, if necessary or desired.
The second step, shown as 20 in FIG. 1, is to provide a grating pattern and the necessary scale factor and address unit values to the MEBES tool. The scale factor value to be applied to the grating pattern for each grating segment to be written onto the mask is established during the design and is known prior to the fabrication of the mask.
TABLE 1Scale Factor jobdeck example**********************CHIP 1,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0738)ROWS 62500/13805.6585CHIP (2,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0738027,GC=1)ROWS 62500/14304.97612775CHIP 3,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0738054,GC=1)ROWS 62500/14804.295011CHIP 4,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0738081,GC=1)ROWS 62500/15303.61514975CHIP 5,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0738108,GC=1)ROWS 62500/15802.936544***CHIP 199,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0743346,GC=1)ROWS 62500/ 112695.034811CHIP 200,(1,PHASEDE-MO-TK,AD=0.125,SF=1.0743373,GC=1)ROWS 62500/ 113194.60102775******************************
Table 1 is an excerpt of a typical jobdeck of the commands issued to a MEBES III or 4500, illustrating the commands instructing it to write the first five and last two of 200 grating segments on a particular substrate. The MEBES jobdeck addresses each segment as CHIP followed by the segment number. Referring to the jobdeck shown in Table 1, PHASEDE-MO-TK is the name arbitrarily given to the particular grating pattern from which the grating segments written to the substrate are modelled, AD is the address unit in microns prior to scaling and SF is the scale factor.
The address unit is chosen at the design stage, as with the scale factors, prior to the fabrication of the mask. AD is an integral divisor of the unscaled pitch of the grating pattern. A smaller value for AD results in increased accuracy and resolution, whereas a larger value results in an improved write time. For an unscaled pitch of 1.0 micron, the address unit is typically either 0.1 micron or 0.125 micron.
The location of the center of the grating segment follows the ROWS command, given as Y/X coordinates on an axis fixed relative to the substrate. As is well known in the art, a typical jobdeck provides all of these values established during the design phase for each of the grating segments in the mask.
Execution of the jobdeck by the lithography tool is the next step, shown in FIG. 1 as 30. In this step, the grating segments that comprise the chirped fiber Bragg grating phase mask are written one-by-one onto the photoresist coated substrate by exposing the photoresist to an image writing element such as an electron beam in a manner well known in the art. Control of the image writing element is carried out internally by the MEBES tool based upon the commands in the jobdeck. As shown in steps 30a-30e, the MEBES tool follows a specific set of procedures when called upon to write a grating segment to the substrate. First, as shown in 30a, the MEBES tool retrieves the scale factor value for the next segment and calculates a new base writing unit for the mask by applying the scale factor to the address unit, 30b. 
In the next step, shown in 30c, the MEBES tool performs a re-registration and recalibration. This time consuming step is necessary whenever the base writing unit is changed. As is known in the art, the command GC=1 shown in Table 1 (applied to CHIPs 2-200) reduces the other recalibrations undertaken by the MEBES tool to the minimum required to obtain properly scaled segments. As shown in 30d, the MEBES tool writes the grating pattern by exposing the photoresist according to the new base writing unit at the axis location defined in the jobdeck.
After writing a segment to the substrate, the MEBES tool checks the jobdeck for the next segment. As shown at 30e of FIG. 1, the MEBES tool will repeat the steps shown at 30a-30d until the last of the grating segments has been written to the substrate and the array is complete. When the mask has been fully exposed, it is processed and used to form a chirped fiber Bragg grating in an optical fiber in the conventional manner as described above, and shown in blocks 40 and 50.
Unfortunately, despite minimizing throughput overhead added due to recalibration of the MEBES tool to its minimum, the repetition of steps 30a-30d still requires that for each successive segment the base writing unit must be redefined to correspond to the new scale factor for that segment. Because the pitch of each segment changes relative to the previous segment in a chirped Bragg grating mask, the repeated recalibrations necessary at steps 30b and 30c can add significant throughput overhead, especially for masks having an array with a large number of segments. This disadvantage can result in fabrication times for a typical 200 segment chirped Bragg grating phase mask to be over 8 hours. From a commercial standpoint, this write time limits both the number of segments and the overall number of index modulations that can be written onto the mask, and ultimately printed to the optical fiber.
Additionally, the technique is not sufficiently accurate for many fiber Bragg grating applications. Stitching and pitch errors have been observed using the scale factor method that result in phase errors and unacceptable levels of a phenomenon known as group delay ripple (GDR) in the chirped Bragg grating ultimately printed on the fiber using the mask. GDR is the wavelength dependent deviation from the theoretical group delay. Group delay is the time delay response curve across the reflected bandwidth of a chirped Bragg grating. GDR is normally reported as the maximum peak to peak deviations from this curve measured in picoseconds. A measurement of GDR indicates the degree to which spatially induced wavelength dispersions are corrected by the fiber Bragg grating. For example, errors typical in chirped fiber Bragg gratings made with masks fabricated using the scale factor method on the MEBES III measure 60 picoseconds of GDR. The MEBES 4500, executing the same jobdeck has produced masks measured at 30 picoseconds of GDR. Although the MEBES 4500 represents an improvement over the MEBES III, neither tool approaches the accuracy needed for critical applications such as those in telecommunications, typically better than 10 picoseconds of GDR.
It is known in the art to reduce GDR in a phase mask for a chirped fiber Bragg grating by using a multipass writing strategy. For example, when writing a grating mask on an e-beam tool such as the MEBES or similar lithography tool using a multipass technique employing four passes, the intensity of the image writing element is reduced to ¼ of the intensity used to expose the substrate during a single pass. By shifting the error boundaries, stitching error is reduced.
Although methods such as the multipass strategy achieve achieving a sufficiently low GDR value in a mask produced by an e-beam or similar lithography tool, application of the technique is rendered commercially impossible using the method of the prior art. At the typical rate discussed above exceeding 8 hours per pass using the scale factor method, the production time for a finished phase mask for a chirped fiber Bragg grating applying the multipass technique can be measured in days. Although other lithography tools may have different write times, the effect of repeated recalibration of basic operational units is similarly time-consuming.
Therefore a need exists for a method for writing chirped fiber Bragg grating phase masks using an e-beam or other lithography tool that significantly reduces write time by avoiding repeated time-consuming recalibration.
A further need exists for a method for writing chirped fiber Bragg grating phase masks presenting a reduced GDR in the fiber Bragg gratings printed therefrom.
While the prior art is of interest, the known methods of the prior art present several limitations which the present invention seeks to overcome.
In particular, it is an object of the present invention to provide a method of achieving scale changes for the image writing element of a lithography tool, including, in particular, a lithography tool for fabricating a phase mask for producing chirped fiber Bragg gratings, that satisfies the above-described needs.
It is another object of the present invention to solve the shortcomings of the prior art.
Other objects will become apparent from the foregoing description.